The present invention relates to a novel process for preparing a carbonaceous material, and more particularly, to modification of carbon black and to a process for preparing and purifying nano-size particulate graphite, a kind of fullerene.
A fullerene which has received attention as new carbon is, chemically, a transformation of carbon black. Comparing the former with the latter with respect to a micro structure; the latter is formed by stacking an infinite honeycomb-shaped network plane at uniform intervals in parallel regularly in three-dimensions; contrary to this, in the case of the former a closed minimum unit system corresponding to a molecule can be strictly defined and the individual system is composed of, in principle, polyhedron-type network comprising arbitrary number of hexagon and 12 pentagons. For such a unique structure, the fullerene has been investigated as a new material of the twenty-first century in each field and it has been expected that the fullerene can be applied in a wide variety of fields from a superconductor, semiconductor and nonlinear optical material down to new type fuel and novel pharmaceutical active substances.
Form of the fullerene can be divided into spherical-type and tube-type. And mono layer-type and multi-layer type are known in each type. An active basic research has been developed everywhere in the world on C60, C70 and mono-layer and multi-layer carbon nano-tubes as objects. Papers of 13,000 and above have been printed and published from 1990 and 1991 when these two forms of fullerene carbon has been isolated for the first time up to the present time. Nobel Prize for chemistry was awarded for discovery of C60. From such facts as above described, strength of impact given by the appearance of fullerene carbon to a scientific and technological field can be surmised. Examining, however, aspects of the applications of the fullerene, it can be said that there are many technical problems to be solved from the standpoint of industrial aspect, for example the production cost of C60 and carbon nano-tube are not yet short of ¥1,000,000 per kg at present, respectively.
The fullerene is included in specific soot, that is, fullerene black obtained by subjecting a graphite electrode to arc-discharge or irradiating graphite with laser in an inactive atmosphere such as helium and so on to vaporize carbon, cooling slowly and aggregating. The form of the fullerene is generally as follows: Mono-layer spherexe2x80x94Fullerene in a narrow sense shown by the general formula Cn, While C60 and C70 are representative for Cn, the arc many cases where 76xe2x89xa6n less than 100 is called as xe2x80x9chigher fullerenexe2x80x9d or 76xe2x89xa6n less than 100 is called xe2x80x9cgiant fullerene.xe2x80x9d The higher fullerene is soluble in an organic solvent and can be extracted and isolated from the fullerene black. C60 is a true sphere having a diameter of 0.7, but the remainder is an ellipsoidal sphere or polyhedron. Multi-layer spherexe2x80x94Concentric multi-layer polyhedron particles having a small cavity therein exist in the fullerene black, which are called as xe2x80x9ccarbon nano-particle.xe2x80x9d Mono-layer tube-typexe2x80x94This takes the form that a mono-layer fullerene is divided into two equal parts and a small wound graphite plate is inserted between them and connected. This can be formed by mixing a certain kind of metal catalyst with graphite by means of arc discharging method to vaporize, which is called as xe2x80x9cmono-layer carbon nano-tube.xe2x80x9d Multi-layer tube-typexe2x80x94This has the structure that several or tens layers of mono-layer carbon nano-tubes having different thicknesses are stacked concentrically. This can be formed in a residue of cathode by arc-discharge on a graphite electrode without a catalyst. In the multi-layer sphere fullerene of the above-described fullerenes, the true sphere-type having no excess space therein has been discovered other than the above-described polyhedron type having a cavity therein. That is to say, if the fullerene black is irradiated with a concentrated electron beam in an electron microscope, a polyhedron-type carbon nano-particle contained therein changes to a substantial true sphere-type multi-concentric structure having no cavity therein. This product is called as xe2x80x9ccarbon nano-onion.xe2x80x9d Since the carbon nano-onion shows substantially perfect sphere, it has been expected to have more interesting performance than the polyhedron-type carbon nano-particle, and it is called as xe2x80x9cultimate fullerene.xe2x80x9d
Each of the concentric polyhedron-type fullerene having a cavity therein, that is xe2x80x9ccarbon nano-particlexe2x80x9d and the concentric true sphere-type fullerene having no cavity can be regarded as a cage transformation of graphite. The latter true sphere-type fullerene is a kind of unstable phase as described below, and it is considered that the latter true sphere-type fullerene changes to a stable polyhedron-type fullerene having no cavity by heat treatment.
However, the studies on nano-size particulate graphite have been undertaken by making use of fullerene black as a raw material. Since it is, however, difficult to separate amorphous carbon or other products contained in the fullerene from the nano-size particulate graphite, the nano-size particulate graphite itself has not been isolated yet. Further, an effective method for preparing the nano-size particulate graphite has not yet developed. It is, therefore, the present condition that the properties of the nano-size particulate graphite have not been clarified yet.
For such reasons as above described, it has been eagerly required to produce nano-size particulate graphite efficiently and in high purity and to clarify its properties.
That is to say, a problem of the present invention is to provide a process for preparing nano-size particulate graphite efficiently and in high purity and to provide high-purified nano-size particulate graphite produced by the process.
The inventor of the present invention has studied devotedly in order to solve the above-described problem. In the process of our studies, soot-like carbon having a grape-like aggregate structure of nano-primary particles formed by stacking irregular concentric spheres in multiple layers was irradiated with high energy beam such as strong electron rays for a short time. As a result, it has been surprisingly observed that the primary particles changes to true sphere-like carbon nano-onion and simultaneously the aggregate is transformed to a state that particles are dispersed.
It has been considered that the carbon black does not take a micro structure having three-dimensional regularity such as graphite, but is not perfect amorphous, and that the carbon black shows broad absorption in the vicinity of low diffraction angle of 20-30xc2x0 in powder X-ray diffraction and has a partial structure in which nano-size carbon flakes comprising multi-nucleus aromatic skeleton are stacked in multiple layers. For this reason, the carbon black is called as low regularity carbon. When such carbon intact is heat-treated at temperatures of 3000xc2x0 C. and above, there is a case where it is graphitized such as pitch coke (easily-graphitization carbon). However, it becomes generally a concentric polyhedron-type micro particle having faces formed by minute graphite layers stacked and having a big cavity therein. The latter is not graphitized, even if it is heated at high temperatures (hard-graphitization carbon). When, however, the micro structure of the carbon black or soot before heat-treatment is well observed, it appears that substantial sphere nano-size particles aggregate in a state of grape and the primary particle has a lamellar structure.
It has been believed from X-ray analysis that such a lamellar structure is formed by depositing plane-like graphite crystallites having an average size of 1.7 nm repeatedly in the direction parallel to a spherical surface. It can be, therefore, said that the above-described discovery by the inventors of the present invention is entirely surprising.
The inventors of the present invention have devotedly studied the structure of low regularity graphite independently from the theory of crystallite graphite which has been conventionally believed to be true. As a result, we have succeeded in producing nano-size particulate graphite efficiently and in high purity and accomplished the present invention.
That is to say, the present invention relates to a process for producing nano-size particulate graphite comprising: irradiating carbon having a micro structure which is a grape-like aggregate of nano-primary particle formed by stacking irregular concentric spheres in multiple layers, said primary particle having no cavity therein and a diameter of approximately 10 to 100 nm with high energy beam.
Further, the present invention relates to the above-described process characterized in that the nano-size particulate graphite is true sphere.
Further, the present invention relates to the afore-mentioned process characterized in that heat-treatment is carried out after irradiation with high energy beam.
Further, the present invention relates to the afore-mentioned process characterized in that nano-size particulate graphite is polyhedron.
Further, the present invention relates to a process for producing nano-size particulate graphite comprising irradiating soot-like carbon which is obtained by incomplete combustion or thermal cracking of a carbon-containing compound such as hydrocarbon, aromatic oil and so on with high energy beam.
Further, the present invention relates to the above-described process characterized in that the soot-like carbon is carbon black.
Further, the present invention relates to the above-described process characterized in that the nano-size particulate graphite is true sphere.
Further, the present invention relates to the above-described process characterized in that heat-treatment is carried out after irradiation with high energy beam.
Further, the present invention relates to the above-described process characterized in that nano-size particulate graphite is polyhedron.
Further, the present invention relates to the above-described process characterized in that the high energy beam is at least one selected from the group consisting of electron rays, gamma rays, X-rays and beam from an ion source.
Further, the present invention relates to nano-size particulate graphite produced by any one of the above-described processes.
While there is a portion which is not yet clear with respect to the mechanism of the formation of nano-size particulate graphite of the present invention, it can be inferred, on the whole, to be as follows:
The afore-mentioned theory of graphite crystallite on carbon black has been considered to be high reliable because it has been derived from the results of X-ray analysis. However, the theory of graphite crystallite is unfavorable, because numerous atomic bonds are left at the ends of the crystallite without being bonded. It is rather natural to consider that the carbon black takes, on the whole, the structure analogous to that of carbon nano-onion having numerous defects, because there are many cases where a five-membered ring is formed when adjacent crystallites are bonded each other. It can be understood that the five-membered ring formed has been probably overlooked by X-ray analysis, because the diffraction strength of X-ray is extremely low relative to the structure around the five-membered ring irregularly distributed. When the carbon black is heat-treated, the heat relaxation proceeds preferentially, because more time is required for elevation of interior temperature. Accordingly, the minute graphite structure grows toward graphite which is the most stable structure of carbon. However, particulate structure can be maintained because the five-membered ring does not disappear by heating. Therefore, the carbon black is supposed to have changed to such hard-graphitized carbon as described above. It is considered that a big cavity inside the carbon results from gas generated by thermal cracking of impurities and the like. However, when the interior temperature is increased within such a short range of time that does not allow relaxation for graphite structure or decomposition of impurities to occur by irradiating the carbon black with high energy beam, it is considered that the carbon black changes toward carbon nano-onion which is near the minimum point of energy on the surface the energy at which is over the energy radiated.